The case demonstrated the difficulties of differential diagnosis and selection of therapy in patients with Parkinson’s disease and schizophrenic spectrum disorders. A clinical case of a 45-year-old male with a 10-year history of paranoid schizophrenia, with hallucinatory paranoid attacks and a steady increase in negative symptoms in the form of apato-abulic defect, is presented. The use of antipsychotics was associated with neuroleptic parkinsonism, well responding to therapy, and reduced when the patient discontinued antipsychotic therapy. However, after 10 years of schizophrenia, Parkinson’s syndrome manifested without neuroleptics; the antipsychotic therapy led to a sharp deterioration in motor status. A neurologist specializing in extrapyramidal disorders made the following diagnosis: Parkinson’s disease, mixed form, Hoehn and Yahr stage 2. According to the neurologist’s recommendation, typical neuroleptics were replaced with quetiapine, and levodopa was added to therapy. During treatment, the parkinsonism symptoms significantly decreased without an increase in psychotic symptoms. The clinical case illustrates the challenges in diagnosing Parkinson’s disease in a patient with schizophrenia and the complexity of treating such comorbidities. When managing such patients, close collaboration between psychiatrists and neurologists is necessary to make an accurate diagnosis and select the optimal therapy. The introduction of the multidisciplinarity principle will make it possible to achieve the fastest diagnosis and selection of balanced therapy in this category of patients.